In work already completed we have provided evidence that the ganglion cells of the isolated rabbit retina are cholinoceptive. A companion study will investigate the retina's release of radioactive acetylcholine. Retinas will be incubated in a flowing medium containing C14 choline during stimulation by flashing light. The labelling medium will then be replaced with control medium and samples of the perfusate collected every 5 minutes. Acetylcholine released will be identified by high voltage paper elctrophoresis, or by scintillation counting of the perfusate. We will evaluate the effects on release of stimulation with light, incubation in medium containing altered concentrations of Mg ions and Ca ions, or incubation with hemicholinium-3. Further experiments will use extracellular recording techniques already developed to evaluate the sensitivity of ganglion cells to putative transmitters other than acetylcholine. An additional study, now near completion, will use the in vitro incubation of retinas from infant rabbits to follow the development of receptive fields between the ages of 10 to 20 days.